Mangroves can cope with sea level rise by increasing soil height

A new report by The Nature Conservancy and Wetlands International shows that mangroves can adapt to rising sea levels by building up soils in some locations, allaying fears that mangroves may be lost as sea levels rise. This is important because mangroves provide risk reduction services against coastal hazards such as waves and storm surges.

Mangroves can protect human lives and property by reducing the impacts of storm surges and waves. However, a major concern has been that mangroves may be lost as sea levels rise, leaving communities more exposed to coastal hazards. This review conducted by The Nature Conservancy and Wetlands International shows that under some circumstances, mangrove soils can build up at similar rates to local rises in sea level, allowing mangroves to survive in situ.

“We need to understand how mangrove soils build up, so that we can maintain suitable conditions for them to do so into the future,” says Nyoman Suryadiputra, Director of Wetlands International Indonesia. “Protecting mangroves is vital for many coastal communities, who rely on them for their livelihoods as well as the coastal defence benefits they provide.”

Past evidence suggests that some mangrove soils have built up at rates between 1 and 10 mm per year. Currently global mean sea level is rising at a rate of 3 mm per year. This suggests that in some places, mangrove soil surfaces will be able to keep up with rising sea levels. This is key, as mangrove trees cannot survive if their aerial roots are submerged for a long period of time.

This latest report highlights the need to maintain, restore or enhance sediment supplies to mangrove areas. The sediments contribute to the build-up of soils, but the supply of sediment to many mangrove areas has been reduced because of dams built on rivers. Mangroves also need protecting from pollution and the felling of trees: the underground roots of healthy trees can push the soil up, while the roots of trees weakened by habitat degradation are less able to hold soils together, potentially leading to erosion and loss of surface soils. Restoring mangrove areas and safeguarding the health of trees can help mangrove soils to build up and so keep pace with sea level rise.

“In some areas, however, mangrove soils may not be able to build up fast enough to keep pace with sea level rise”, alerts report lead author Dr Anna McIvor of The Nature Conservancy. “In these areas, local planners should allow space for mangroves to colonise landward areas as sea levels rise. This will help to ensure that mangroves continue to reduce risks from coastal hazards into the future, benefiting local communities”.

Some mangrove forests have survived in the same location for thousands of years by building up soils beneath them as sea levels rose. In Twin Cays in Belize, mangroves have created a layer of old roots and sediments that is up to eight metres thick in some places. By building up soils, mangroves also help to lock up greenhouse gases such as carbon dioxide, and this provides another reason for protecting mangroves and their soils from degradation and loss.

Dr Mark Spalding, senior marine scientist at The Nature Conservancy said, “It is essential that we protect mangrove forests as they provide many vital services, not just coastal defence, but also fisheries and carbon storage.”

Notes to editors

1. The Nature Conservancy is a leading conservation organization working around the world to protect ecologically important lands and waters for nature and people. The Conservancy and its members have protected more than 480,000 sq km of land and is engaged in more than100 marine conservation projects in 24 countries, including the Caribbean and the Coral Triangle. The Conservancy is also developing policy and decision support tools for ecosystem-based adaptation and hazard reduction in the coastal zone.
2. Wetlands International is working to sustain and restore wetlands, their resources and biodiversity. Wetlands International is the only global non-profit organisation dedicated to the conservation and restoration of wetlands. It works through a network of 16 offices and many partners and experts to achieve its goals.
3. The Cambridge Coastal Research Unit is based in the Department of Geography in the University of Cambridge. It aims to provide high quality scientific research to underpin sustainable coastal management.
4. The report “The response of mangrove soil surface elevation to sea level rise” can be found here.
5. In earlier reviews this same team drew together compelling evidence of the potential role of mangroves in attenuating regular waves and storm surges:
a. McIvor, A.L., Möller, I., Spencer, T. and Spalding. M. (2012) Reduction of wind and swell waves by mangroves. Natural Coastal Protection Series: Report 1. Cambridge Coastal Research Unit Working Paper 40. Published by The Nature Conservancy and Wetlands International. 27 pages. ISSN 2050-7941.
b. McIvor, A.L., Spencer, T., Möller, I. and Spalding. M. (2012) Storm surge reduction by mangroves. Natural Coastal Protection Series: Report 2. Cambridge Coastal Research Unit Working Paper 41. Published by The Nature Conservancy and Wetlands International. 35 pages. ISSN 2050-7941.
6. The world’s coastal margins are among the most densely populated and intensively used places on earth. These populations and coastal lands can be at risk from natural hazards such as waves, storms and tsunamis. With the expansion of human populations the numbers of people at risk are increasing. Mangroves are impacted as populations expand, reducing their ability to provide crucial coastal defence.